Coating film for zinc surfaces and method of making the same



Patented June 27, 1 944 7 COATING mm FOR 'zmo summons AND METHOD orMAKING THE SAME Joseph J. Waters, Long Island City, N. Y., assignor toEtched Products Corporation, Long Island City, N. Y., a corporation ofNew York No Drawing. Application November 6, 1942,

' Serial No. 464,938

3 Claims.

This invention relates to improvements in and methods for makingcoatings for metal surfaces of zinc or zinc alloys.

Up to the present t me, considerable difficulty has been encountered inthe coating of sheet zinc, or galvanized iron which incorporates zinc,

by virtue of the apparent loss of adhesiveness of the lacquer coatingupon drying. Several explanations have been offered as to the cause ofthis phenomenon; for example, some authorities indicate that theshrinkage of the nitro-cellulose coating, as the solvents therein becomedry upon aging, causes a loss of adhesiveness of the surface of thenitro-cellulose body; others indicate that lacquers are essentiallynon-polar to the zinc of the metal surface, so that as the volatilesolvents, which ar more polar to the metal, evaporate, the now morebrittle nitro-cellulose becomes even less polar and thereby furtherloses its adhesive properties; still others, on the one hand, attributethe break-down of coating films, in contact with zinc, to the formationof occluded oxygen which, on coming to the surface of the metal at thefilm layer, undermines the coating film and destroys its adhesion,while, on the other handjo thers theorize to the effect that there is adecomposition of the nitro-cellulose in which free acid. liberatedtherefrom, reacts with the zinc surfaces, with the zinc acting as acatalyst, with the result that a progressive electrolytic action reducesthe polarity and the adhesion of the nitro-cellulose to the zincsurface.

Various mechanical expedients, such as tumsolvents which controlsadhesion of a lacquer to a zinc surface, but rather that the lacquer, byvirtue of the absence of a metallic salt, is ab initio and alwaysremainsv substantially non-' polar to a zinc surface and on this basis,with the desire to increase the polarity of the lacquer to zinc, I havetreated the lacquer, before application, as follows:

I have added to a lacquer of any color a relafine mesh and permittedthe'zinc dust to stand bling or blasting; chemical expedients, such asetching or coating with various organic salts;

outside coating, introduces an undesired double thickness of coatingsand the'expense in both time and material of two coating operations,particularly-in the manufactur of etched indicia plates, or the like,where it is essential not only that a single lacquer layer or coating beapplied,

but that such coating be very thin.

In contradistinction to some of the theories advanced in the literature,itis my belief that it is not a matter of the drying of nitro-cellulosefor a shortperiod of time; whereupon the zinc dust substantiallyentirely settled out; There- I after, I decanted the lacquer solutionfrom the settled zinc dust residue. The decanted lacquer,

when applied as the only coating to the zinc sur-' face, has been foundto firmly adhere, even after aging of several months. Mypreferredpreparations comprise one ounce of zinc dust of 300 mesh and fin'er to agallon of 'nitro-cellulose lacquer, and the settling out of-the zincdust commences immediately and, at the end of twelve (12) hours,hassettled out in an amount of more than ninetynine (99%) per cent,after which period, the thug treated lacquer is decanted.

As above stated, I have found that substantially all of the 'zinc dustapplied to the lacquer settles out as a residue, and consider thatessentially one of three phenomena occurs: (a) a zinc salt is formedwith some of the acid in the lacquer, thereby rendering the same morepolar to the zinc by virtue of the presence of the zinc salt; or (b) thezinc dust reduces some of the constituents'of the lacquer, which areoxidizers or have a tendency to liberate oxygen upon aging; or (c) 'thezinc dust possibly serves as an absorptive medium for that constituentof the lacquer which normally initially or progressively renders thenitro-cellulose lacquer non-polar to the zinc surface being coated. l t

In any case, I do not consider that I have ascertained the actualchemical, physical or electrochemical phenomenon that occurs by virtueof my treatment of the nitro-cellulose lacquer with the zinc dust asabove definedj'other than to have discoveredthat such treatment rendersthe nitrocellulosefirmly adhesive as a coating to. a zinc surface.

Various changes and modifications may be made to the details of theinvention without de- I claim: 1. A method for treating a pyroxylinlacquer ,to render the same adhesive to zinc surfaces,

which comprises adding zinc dust to the lacquer and, after permittingthe zinc dust .to settle, decanting therefrom the thus treated pyroxyiinlacquer.

2. A coating for and rendered adhesive to zinc metal surfaces comprisingthe decanted portion of an intermixture of pyroxyiin lacquer and zincdust from which the zinc dust has settled out.

